Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets

Abstract

Abstract. We report important results of the ®rst campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two dierent types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The ®rst event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also con®rmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O + recombination due to an elevated T i , which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200±300 km in width. They were seen principally with the Sondrestrom, and Super-DARN radars. Enhanced ion temperature (T i ) was also observed by the Sondrestrom and EISCAT radars. These channels of high T i were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma¯ow is important to de®ne the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The eect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.

Statistics of GPS ionospheric scintillation and irregularities over polar regions at solar minimum

“…The dominant mechanisms for patch formation are still unknown. Two of the most prominent mechanisms at present are sporadic chopping of a pre-existing tongue of high density plasma (Valladares et al 1999), which lead to density depletions in a preexisting tongue of high density plasma entering the polar cap from noon collocated with jets of high plasma velocity, and sporadic injection of high density plasma (Lockwood and Carlson 1992), which lead to density enhancements associated with high plasma velocity flows.…”

Earth's ion upflow associated with polar cap patches: Global and in situ observations

“…Polar cap patches are formed by ionospheric dynamics in the “cusp region” [ Crowley , ; Carlson , ; Lockwood and Carlson , ; Valladares et al ., ; Rodger et al ., ; Zhang et al ., ]. They appear as islands of high‐density plasma in a background that may be less than the patch density by 50% or more [ Crowley , ; Carlson , ].…”

Simultaneous observations of traveling convection vortices: Ionosphere‐thermosphere coupling

“…Dynamics associated with ionospheric plasma during TCV events have also been reported. Observations using the European Incoherent Scatter (EISCAT) and Sondrestrom radars show significant plasma density erosion due to recombination of oxygen caused by increased frictional heating (T i ) within the vortex [ Valladares et al , ]. Lühr et al [] used EISCAT to measure the conductivity associated with the vortices and found increased conductivity in the downward field‐aligned current region.…”